I saw that Paul on Facebook.... not good
I must admit I was looking at my old TripleX, (with the same back-end arrangement) the width of the rear plate is the same as the AC chassis, so apart from maybe chamfering the edge a bit, I don't see the need to trim that part down? The 'T' peice on the other hand, does need cutting out to clear the front of the wishbones. The shocks limit the droop more than we expect I think - the wishbones don't drop down much further than horizontal.
I've been mocking up the gearbox to see what needs doing to get it to fit 100% - I've clearanced the rear tower (shaved off a few thou with a scapal blade) & now it sits snug over the trans housing. As Legacy555 has done, I will also need to thin down the motor plate mounting face - i'm going to take off as much as I dare without breaking into the layshaft bearing pocket, but i've got some spare long 4/40 screws, so I might, if need be, have to thin down the brass L bracket to get the hole to align & cut down the spare screws if the originals still aren't long enough to reach through the gearbox. I think this extra thickness is a bit of a manufacturing error - the PSI chassis is designed to use a standard idler equipped Losi box just like the CR2, with the extra offset of the motor plate from using the PSI 4 gear box the slipper back plate is pretty much virtually touching the motor plate.